11.2.1 Cyclic Voltammetry (CV)
Cyclic voltammetry (CV) is one of the most versatile electroanalytical techniques for studying electroactive species. Since it is relatively easy to use and gives a good overview of the redox processes occurring in a certain potential range, it is frequently used within all fields of chemistry. The graph of a cyclic voltammogram reflects the reaction occurring at the electrode surface as the potential is cycled. According to Nernst’s equation (equation 1), the ratio of COX/CRED will change when potential on the working electrode is changed. The value of the formal potential E° for a reversible reaction can be calculated using the anodic and cathodic peak potential (Epa and Epc).
E° = (Epc + Epa)/2 (eq. 1) The obtained reduction and oxidation peak potentials by CV measurements allow the estimation of the LUMO and HOMO levels of the investigated compounds.
A typical three-electrode cell suitable for studies of materials includes a reference electrode, a counter electrode, and a working electrode.
Figure 95. Schematic representation of cyclic voltammetry measurement cell.
CV measurements were carried out using millimolar solutions in highly dried tetrahydrofuran (THF) at room temperature. A glassy carbon disk electrode (0.2 cm2) coated with a thin polymer film was used as working electrode. A platinum wire was used as a counter electrode and a silver wire in solution of AgNO3 was used as a quasi-reference electrode. Each measurement was calibrated with an internal standard, ferrocene/ferrocenium (Fc) redox
system. The HOMO and LUMO values were determined from the value of -4.8 eV for Fc with respect to vacuum level. Scanning rates in the range of 50 to 500 mV·s-1 were used.
11.2.2 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry measurements were carried out using a Diamond DSC from Perkin-Elmer. The heating/cooling rate was 10 K·min-1 for all experiments. Sample weights in the range of 15 to 25 mg were used in a 70 µl pellets.
11.2.3 Electroluminescence Emission Spectroscopy
Fluorescence spectra and emission spectra of OLEDs were measured using a Shimadzu RF-5301 PC spectral fluorometer. The excitation beam optical path way was closed. The OLED was placed right in front of the emission beam path way and the device was biased using a Grundig PN 300 power source. Voltage dependent EL spectra were carried out. For a certain bias, the corresponding electroluminescence could be detected and emission spectra were taken.
11.2.4 Elementary Analysis
Elementary analysis of the synthesised polymers was carried out at the Mikroanalytisches Labor Pascher (an der Pulvermühle 1, 53424 Remagen, Germany). This Labour provides the possibility to detect the amount of Cu and Fe in solid-state materials.
11.2.5 Flash Column Chromatography
Column chromatography was performed by using silica gel (KIESELGEL 60; Merck) or alumina oxide (Alumina N, Akt. I; ICN Biomedicals GmbH) in 2, 4 or 6 cm thick glass column. First the column was filled up with adsorbent, and then the eluent passed through the column with a force to remove remaining air in adsorbent. Then the substance to be purified was dissolved in a minimum volume of eluent on top of the column followed by elution.
During the chromatography the pressure in the column was applied in such a way to get eluent flow in the range of 5 cm of column height pro 60 sec. In a second step purified components was identified by TLC. The solvent was removed using a rotation evaporator.
11.2.6 Fluorescence Spectroscopy
Fluorescence spectra in both solution and film were measured using a Shimadzu RF-5301 PC spectral fluorometer. Excitation wavelength used in excitation beam was equal to
corresponding wavelength of maximum absorption from UV-Vis spectroscopy. For measuring fluorescence spectra of solutions the same solutions from UV-Vis spectroscopy were used. On condition that fluorescence in solid state was sufficient, measurements were carried out using thin films on precleaned glass substrates.
11.2.7 Fourier Transformed Infrared Spectroscopy (FT-IR)
Infrared spectra were measured using a BioRad Digilab FTS-40 (FT-IR). Measurements were carried out by using a potassium bromide (KBr) pellet for substances in a solid state and by using sodium chloride pellet for substances in liquid state (16 scans; resolution: 4 cm-1).
11.2.8 Mass Spectrometry
Mass spectra (Finnigan MAT 8500, Varian MAT 112 Spectrometer) were taken at a ionization energy of 70 eV (Electron-Impact).
11.2.9
1H-NMR-Spectroscopy
Proton Nuclear Magnetic Resonance Spectroscopy was carried out using a AC 250 spectrometer from Bruker (250 MHz) in deuterated chloroform (CDCl3), benzene (C6D6), tetrahydrofuran (d4-THF) or dimethyl sulfoxide (d6-DMSO). Chemical shifts (δ) were reported in ppm downfield from tetramethylsilane (TMS). All measurements were carried out at room temperature. The following abbreviations were used for signal characterization: d (doublet), t (triplet), m (multiplet, signals without resolution). Signals from spin systems of higher order showing simple line splitting were interpreted like spin systems of 1st order.
11.2.10 Size Exclusion Chromatography (SEC)
Size exclusion chromatography (SEC) measurements were performed on a Waters system with Waters 410 RI and Water 440 UV detectors against polystyrene standards by using THF containing 0.25 wt. % of tert-butyl ammonium bromide as eluent at room temperature. The molecular weights and the molecular weight distributions were calculated on the basis of monodispersed polystyrene standards.
11.2.11 Sublimation
Sublimation was performed at a pressure of less than 3.0·10-5 mbar by heating the substance to be purified with an oil bath. High vacuum turbomolecular pump PT151 Kit from Leybold
was used. Cooling of the condensing finger of sublimation apparatus was provided by water or water/ice mixture respectively.
11.2.12 Thermo-Gravimetric Analysis (TGA)
Thermo-gravimetric measurements were carried out using a thermoanalysis apparatus TGA/SDTA851e from Mettler Toledo. Samples for the measurements were prepared by filling alox crucibles. Measurements were performed at a heating rate of 10 K·min-1 under a nitrogen flow rate of 75 cm3·min-1 in the temperature range from 30 to 650 K.
11.2.13 Thin-Film Layer Thickness Measurement
Thickness of organic materials spin-coated or vapour deposited onto ITO substrates was in the range of 20-300 nm. Thickness measurements were carried out using a surface profilometer Decktak 3030 ST. The samples for measurement were prepared by scratching thin film using a scalpel. The layer thickness was taken as detected height difference.
11.2.14 Thin Layer Chromatography (TLC)
POLYGRAM SIL G/UV254 and POLYGRAM ALOX N/UV thin layer chromatography cards were purchased from Carl Roth GmbH. Substances were appliquéd on these cards from solution and separated using the given solvent combination respectively.
11.2.15 UV-Vis Spectroscopy
Absorption spectra were measured with a spectrophotometer (Hitachi U-3000). Solution spectra were measured at a concentration varying between 10-4 to 10-6 M using chlorobenzene, chloroform, cyclohexane, dioxane or THF (all purified according to standard procedures) as solvent. UV-Vis cuvettes with a thickness of 1 cm were used. Extinction coefficient ε is was calculated by equation 2:
ε = (eq. 2)
where OD is an optical density, c – concentration and d – cuvett thickness.
c · dOD
Thin film solid-state spectra were also carried out. Thin films were prepared either by solution casting or physical vapour deposition on ITO substrates. Film thickness was in the range of 20 to 300 nm. Films were characterised by absorption coefficient α, which was calculated by equation 3:
α = 2.303 · OD (eq. 3) c · d